Central processing units (host processors) transfer data to/from and control the operation of peripheral devices, such as direct access storage devices (DASD) and printers (collectively referred to as input/output or I/O devices), with signals transmitted over I/O channels. For many years, I/O channels employed with large computer systems have been based upon parallel channel architecture and have used copper wire.
To meet the demands for higher performance and to overcome certain limitations of conventional parallel architecture, IBM Corporation introduced Enterprise Systems Connection Architecture, also referred to as "ESCON", (both trademarks of IBM Corporation). I/O devices in an ESCON environment are interconnected with the host with fiber optic cables carrying serial control and data signals. Advantages of such a system include, among others: higher transmission rates, more flexibility in the physical location of I/O devices relative to the host (up to nine kilometers, or more, in contrast to about 400 feet in typical copper wire, parallel architectures), lighter and less bulky cables, and more centralized and flexible systems management of I/O configurations.
Information (representing both data and control) is transmitted on an ESCON channel as a series of transactions or frames. Each transaction includes at least a destination address, a command and data. More detailed information pertaining to ESCON architecture can be found in "Role of the DASD Storage Control in an Enterprise Systems Connection Environment" by Grossman (IBM Systems Journal, Vol. 31, No. 1, 1992, pp 123-146).
A large computer system may typically include multiple (if not many) I/O interfaces and ensuring correct I/O interface performance requires a complete understanding of all I/O activity, whether the system is in a non-ESCON parallel environment or in an ESCON environment. Such an understanding can only be obtained by tracing or monitoring all of the attached I/O channels. While a logic analyzer can be used to monitor discrete signal states on a single channel or, perhaps, a few channels, a logic analyzer typically does not translate the signal states into easily understandable, higher level information and also does not chronologically interleave signals from different channels for display in a time-correlated format.
One device developed, called a general purpose tracer (GPT) tool interconnects between a host and attached I/O devices to transparently monitor up to eight parallel I/O channels. The GPT interleaves up to eight traces on a single display (a computer screen or printer) and can also store trace results in internal memory and/or on an external storage device, such as a tape cartridge. Tracing and displaying all interface activity with a single tool reduces diagnostic time compared with tracing each interface with a separate instrument (such as logic analyzer) and then time-correlating the activity of the various channels. Moreover, the GPT is particularly useful in diagnosing problems involving I/O activity which switches between different interfaces. However, the GPT has only been able to process 8-bit parallel electrical signals and has not heretofore accommodated serial optical signals.